This application claims priority from Korean Patent Application No. 2002-70662, filed on Nov. 14, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a planar lens and a method for fabricating the same, and more particularly, to an easy-to-fabricate planar objective lens capable of compensation of chromatic aberration, as an essential part of an optical pickup that is used in an optical disc drive (ODD) to write information in and/or reproduce information from an optical disc, such as a compact disc (CD) or a digital versatile disc (DVD), and an easy method for fabricating the planar microlens.
2. Description of the Related Art
Objective lenses are used to write information in an optical disc by focusing a laser beam emitted from a semiconductor laser used as a light source on a recording surface of the optical disc and to read information from an optical disc by condensing light reflected from its recording surface toward a photodetector. Conventionally, objective lenses for use in writing and reading information with respect to optical information storage media have been fabricated by mechanically cutting and polishing solid glass or plastics to a desired shape. Another conventional methods for fabricating such objective lenses use compression molding or injection molding in which objective lenses are fabricated from molten or semi-molten glass or plastics using molds formed by mechanical processing.
To fabricate an objective lens by compression molding, as shown in FIG. 1, upper and lower molds 2a and 2b, which are located above and below a target object to be manufactured, i.e., a lens 1, respectively, a ring 3 interposed between the upper and lower molds 2a and 2b, and a sleeve 4 as a support for the upper and lower molds 2a and 2b are necessary. These upper and lower molds 2a and 2b are commonly manufactured by a high precision lathe with a diamond wheel. However, this mold processing method is limited as to processing a curved mold surface corresponding to a spherical or aspheric lens surface. In particular, to form a spherical or aspheric lens surface, a mold needs to be machined for a concave surface that matches the desired spherical or aspheric lens surface. However, it is difficult to machine a concave mold for lenses under about 1 millimeter in diameter with the mechanical mold machining method, because cutting tools and the curvature of diamond wheels therefore are limited.
Recent trends toward portable, miniature, and high-density optical discs necessitate smaller optical pickups compatible with these miniature optical discs. Smaller optical pickups should be assembled with smaller constituent optical elements, such as a laser diode, a collimator, a mirror, an objective lens, a photodetector, and the like. However, it is more difficult to manufacture and assemble smaller optical elements, especially with microlenses having a diameter of 1 mm or less because of difficulties in mold machining therefor, as described above. Due to the limitation in mold machining, the smallest known lenses manufactured thus far, for example, by injection molding with a numerical aperture (NA) of 0.85, have a diameter of about 4 mm. Even though such microlenses of 1 mm or less in diameter can be manufactured, it is difficult to handle such a small microlens and to assemble an optical pickup with the microlens.
General objective lenses have two opposing surfaces that are spherical or aspheric and which protrude closer to an optical disc, compared to other adjacent optical elements, so they are likely to be abraded or damaged when collided with the optical disc.
To eliminate these problems, there have been suggested techniques for fabricating objective lenses in an array in a planar substrate using, for example, conventional semiconductor manufacturing processes. These methods are advantageous for mass production and enable easy assembling of optical pickups at low costs.
FIGS. 2A through 2D illustrate a process of fabricating a microlens array suggested by Akira Kouchiyama et al. (Japanese J. Appl. Phys., Part 1, Vol. 40, No. 3B, p. 1792, 2001).
Briefly, in the method suggested by Akira Kouchiyama, a glass substrate 5 is coated with a photoresist 6, as shown in FIG. 2A. After patterning, as illustrated in FIG. 2B, the resulting photoresist pattern 6′ is reflowed by heating to a predetermined temperature, for example, about 150° C. Through this reflow process, a hemispheric photoresist pattern 6″ is formed, as shown in FIG. 2C. Next, reactive ions are supplied into a plasma etching chamber to etch the substrate 5 through the hemispheric photoresist pattern 6″ so that a microlens array is formed in the glass substrate 5, as shown in FIG. 2D. As the photoresist pattern 6 is reflowed by heating, the photoresist pattern 6′ becomes hemispheric by surface tension generated during the reflow. As the plasma dry etching is performed on the glass substrate 5 under appropriate conditions with the hemispheric photoresist 6″ serving as a mask, the hemispheric shape of the mask is transferred into the glass substrate 5. The resulting microlens array has a spherical refractive surface.
In this microlens array manufacturing method by plasma etching, it is difficult to uniformly deposit the photoresist to a sufficient thickness for an objective lens of appropriate thickness so that the sag height of the resulting microlens is limited. In addition, since the hemispheric photoresist pattern formed by reflowing is directly transferred into a glass substrate, the method cannot be applied for general aspheric lenses excluding spherical lenses. Dry etching applied to manufacture the microlens array takes a comparatively longer duration.
According to a microlens manufacturing method suggested by Masahiro Yamada et al., Technical Digest of ISOM/ODS 2002, p. 398, a substrate with a cavity for a lens used for optical control is manufactured by injection molding, and the cavity is filled with a high refractive index material and subjected to polishing to form a planar lens.
Masahiro's method requires a mold of a desired shape to form the cavity in the substrate. Unlike general molding processes where a cavity is formed in a substrate to correspond to a convex surface of a desired lens, a mold shaped like a plug or insert with a convex surface that matches a desired cavity of a substrate is formed by mechanical machining. Over the substrate with the cavity formed through injection molding using the plug-like mold, a high refractive index material is deposited to a predetermined thickness by sputtering and polished such that the high refractive index material remains only in the cavity of the substrate. The cavity filled with the high refractive index material substantially acts as a lens.
This method is suitable to manufacture single microlens but not for an array of multiple lenses. Also, the thin film deposition technique, i.e., sputtering, used in the method takes a long duration for a lens layer having an appropriate thickness. Furthermore, it is also time consuming to polish the deposited high refractive index material lens layer.